1. Field of the Invention
This invention relates to a technique for the production of large area, high quality freestanding (FS) nonpolar and semipolar nitride substrates.
2. Description of the Related Art
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These compounds are referred to herein as Group III nitrides, or III-nitrides, or just nitrides, or by (Al,Ga,In)N, or by Al(1-x-y)InyGaxN where 0≦x≦1 and 0≦y≦1. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the würtzite structure exhibits inherent piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on nonpolar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are equivalent to one another so the bulk crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN are the {11-20} family, known collectively as a-planes, and the {10-10} family, known collectively as m-planes. Unfortunately, in spite of advances made by researchers in nitride community, heteroepitaxial growth of high quality nonpolar and semipolar GaN and high performance device fabrication remain challenging and have not yet been widely adopted in the III-nitride industry. On the other hand, despite the success in high performance devices homoepitaxially grown on high quality nonpolar and semipolar freestanding (FS) GaN substrates, the narrow substrate area makes it challenging to widely adopt into the III-nitride industry.
The other cause of polarization is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al, In, Ga, B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer lattice matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
The advantage of using nonpolar or semipolar planes over c-plane nitrides is that the total polarization will be zero (nonpolar) or reduced (semipolar). There may even be zero polarization for specific alloy compositions on specific planes, for example, semipolar planes. The present invention satisfies the need for enhanced area nonpolar and semipolar substrates.